Power conversion circuit, inverter, and control method

ABSTRACT

A power conversion circuit includes a switching network, a control circuit, a filter circuit, and a direct current side circuit; and the filter circuit includes a first power inductor, a common mode choke, a first differential mode filter capacitor, a first common mode filter capacitor, and a second common mode filter capacitor. The first power inductor includes a first winding and a second winding, and the common mode choke includes a third winding and a fourth winding; a first end of the first winding and a first end of the second winding are separately connected to the switching network, a second end of the first winding and a second end of the second winding are respectively connected to a first end of the third winding and a first end of the fourth winding.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2019/126910, filed on Dec. 20, 2019, which claims priority toChinese Patent Application No. 201910310890.9, filed on Apr. 17, 2019.The disclosures of the aforementioned applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

The embodiments relate to the field of photovoltaic technologies, and inparticular, to a power conversion circuit, an inverter, and a hybridmodulation control method.

BACKGROUND

In a grid-connected photovoltaic power generation system, an inverter isa key device, and conversion efficiency and performance of the inverterdirectly determine a profit of the power generation system.

To reduce costs of the inverter, a non-isolated architecture is used formost current inverters, and a dual-stage relay is used to implementdirect grid-connected photovoltaic power generation. However, in asingle-phase power grid system, one end of a power grid is usuallygrounded at a remote end through a transmission line. In this case, aphotovoltaic panel, an inverter, and the power grid form a throughcurrent loop through the earth. This loop causes a serious leakagecurrent problem.

This leakage current not only causes a personal safety problem, but alsoincreases a loss on the inverter and reduces conversion efficiency ofthe inverter.

SUMMARY

Embodiments provide a power conversion circuit to improve conversionefficiency of an inverter, and effectively reduce a common mode leakagecurrent. The embodiments further provide a corresponding inverter and acorresponding hybrid modulation control method.

According to a first aspect, a power conversion circuit is provided, andmay include:

a switching network, a control circuit, a filter circuit, a directcurrent side circuit, and an alternating current side circuit, where theswitching network is connected to the direct current side circuit, theswitching network is connected to the control circuit, the switchingnetwork is connected to the filter circuit, and the filter circuit isconnected to the alternating current side circuit;

the control circuit is configured to control the switching network;

the filter circuit includes a first power inductor, a common mode choke,a first differential mode filter capacitor, a first common mode filtercapacitor, and a second common mode filter capacitor;

the first power inductor includes a first winding and a second winding,and the common mode choke includes a third winding and a fourth winding;

both a first end of the first winding and a first end of the secondwinding are separately connected to the switching network, a second endof the first winding is connected to a first end of the third winding,and a second end of the second winding is connected to a first end ofthe fourth winding;

two ends of the first differential mode filter capacitor arerespectively connected to the second end of the first winding and thesecond end of the second winding;

a first end of the first common mode filter capacitor is connected to asecond end of the third winding, and a second end of the first commonmode filter capacitor is connected to the direct current side circuit byusing a low impedance circuit; and

a first end of the second common mode filter capacitor is connected to asecond end of the fourth winding, and a second end of the second commonmode filter capacitor is connected to the direct current side circuit byusing a low impedance circuit.

The low impedance circuit in a case in which the second end of the firstcommon mode filter capacitor is connected to the direct current sidecircuit by using the low impedance circuit may be referred to as a firstlow impedance circuit. The low impedance circuit in a case in which thesecond end of the second common mode filter capacitor is connected tothe direct current side circuit by using the low impedance circuit maybe referred to as a second low impedance circuit. The first lowimpedance circuit and the second low impedance circuit may be a same lowimpedance circuit, or may be different low impedance circuits.

It can be understood from the first aspect that, before the firstdifferential mode filter capacitor is connected to the common mode chokein the filter circuit, a loss caused when a high-frequency currentcomponent flows into the common mode choke can be effectively avoided,thereby improving conversion efficiency of the power conversion circuit.In addition, both the second end of the first common mode filtercapacitor and the second end of the second common mode filter capacitorare connected to the direct current side circuit, to provide a lowimpedance loop of a common mode current, thereby effectively reducing acommon mode leakage current from the power conversion circuit to anoutput port.

In a possible implementation, with reference to the first aspect, in afirst possible implementation, the low impedance circuit is a zeroimpedance circuit, or the low impedance circuit includes one resistor orat least two resistors in series.

In a possible implementation, with reference to the first aspect or thefirst possible implementation of the first aspect, in a second possibleimplementation, the filter circuit further includes a second powerinductor, the second power inductor includes a fifth winding and a sixthwinding, the second end of the third winding is connected to a first endof the fifth winding, the second end of the fourth winding is connectedto a first end of the sixth winding, and a second end of the fifthwinding and a second end of the sixth winding are connected to thealternating current side circuit.

In a possible implementation, with reference to the first aspect or thefirst or the second possible implementation of the first aspect, in athird possible implementation, the direct current side circuit includesa positive busbar, a busbar capacitor, and a negative busbar, and twoends of the busbar capacitor are respectively connected to the positivebusbar and the negative busbar. In a possible implementation, withreference to the third possible implementation of the first aspect, in afourth possible implementation, both the second end of the first commonmode filter capacitor and the second end of the second common modefilter capacitor are separately connected to the positive busbar. In apossible implementation, with reference to the third possibleimplementation of the first aspect, in a fifth possible implementation,both the second end of the first common mode filter capacitor and thesecond end of the second common mode filter capacitor are separatelyconnected to the negative busbar. In a possible implementation, withreference to the third possible implementation of the first aspect, in asixth possible implementation, the second end of the first common modefilter capacitor is connected to the positive busbar, and the second endof the second common mode filter capacitor is connected to the negativebusbar.

In a possible implementation, with reference to the third possibleimplementation of the first aspect, in a seventh possibleimplementation, the busbar capacitor includes a positive busbarcapacitor and a negative busbar capacitor, a first end of the positivebusbar capacitor is connected to the positive busbar, a second end ofthe positive busbar capacitor is connected to a first end of thenegative busbar capacitor, and a second end of the negative busbarcapacitor is connected to the negative busbar; and the second end of thefirst common mode filter capacitor and the second end of the secondcommon mode filter capacitor are connected to a middle point between thepositive busbar capacitor and the negative busbar capacitor.

In a possible implementation, with reference to any one of the firstaspect or the first to the seventh possible implementations of the firstaspect, in an eighth possible implementation:

the switching network includes a first converter bridge arm and a secondconverter bridge arm, the first converter bridge arm includes a firstswitching device and a second switching device, and the second converterbridge arm includes a third switching device and a fourth switchingdevice;

the control circuit controls the first switching device and the secondswitching device of the first converter bridge arm through a first sinemodulated wave and a first carrier;

the control circuit controls the third switching device and the fourthswitching device of the second converter bridge arm through a secondsine modulated wave and a second carrier; and

bipolar modulation is performed on the first carrier and the secondcarrier in a first preset angle range, unipolar modulation is performedin a second preset angle range, the first preset angle range is setbased on a direct current bias point of the first sine modulated wave orthe second sine modulated wave, the second preset angle range is anangle other than the first preset angle range in a sine wave period ofthe first sine modulated wave or the second sine modulated wave, and aswitching frequency of the bipolar modulation is higher than a switchingfrequency of the unipolar modulation.

In the eighth possible implementation, a direct current bias means thatan alternating current has a direct current component. The sinemodulated wave may have one direct current bias point. If a directcurrent bias value is 0, the direct current bias point is a zerocrossing point, or if a direct current bias value is not 0, a value ofthe direct current bias point is the direct current bias value. It canbe understood from the eighth possible implementation that a unipolarand bipolar hybrid modulation scheme is used, to further improveconversion efficiency of the power conversion circuit and further reducea leakage current.

In a possible implementation, with reference to the eighth possibleimplementation of the first aspect, in a ninth possible implementation,

the first preset angle range includes (−α, β), a value of −α and a valueof β are adjusted based on state information, and the state informationincludes a voltage of the positive busbar, a voltage of the negativebusbar, and a voltage of the alternating current side circuit.

According to a second aspect of the embodiments, a power conversioncircuit is provided, and may include:

a switching network, a control circuit, a filter circuit, a directcurrent side circuit, and an alternating current side circuit, where theswitching network is connected to the direct current side circuit, theswitching network is connected to the control circuit, the switchingnetwork is connected to the filter circuit, and the filter circuit isconnected to the alternating current side circuit; the control circuitis configured to control the switching network;

the filter circuit includes a first power inductor, a common mode choke,a first differential mode filter capacitor, a first common mode filtercapacitor, and a second common mode filter capacitor;

the common mode choke includes a third winding and a fourth winding;

a first end of the first power inductor is connected to the switchingnetwork, a second end of the first power inductor is connected to afirst end of the third winding, and a first end of the fourth winding isconnected to the switching network;

a first end of the first differential mode filter capacitor is connectedto the second end of the first power inductor, and a second end of thefirst differential mode filter capacitor is connected to the first endof the third winding;

a first end of the first common mode filter capacitor is connected to asecond end of the third winding, and a second end of the first commonmode filter capacitor is connected to the direct current side circuit byusing a low impedance circuit; and

a first end of the second common mode filter capacitor is connected to asecond end of the fourth winding, and a second end of the second commonmode filter capacitor is connected to the direct current side circuit byusing a low impedance circuit.

The low impedance circuit in a case in which the second end of the firstcommon mode filter capacitor is connected to the direct current sidecircuit by using the low impedance circuit may be referred to as a firstlow impedance circuit. The low impedance circuit in a case in which thesecond end of the second common mode filter capacitor is connected tothe direct current side circuit by using the low impedance circuit maybe referred to as a second low impedance circuit. The first lowimpedance circuit and the second low impedance circuit may be a same lowimpedance circuit, or may be different low impedance circuits.

It can be understood from the second aspect that, before the firstdifferential mode filter capacitor is connected to the common mode chokein the filter circuit, a loss caused when a high-frequency currentcomponent flows into the common mode choke can be effectively avoided,thereby improving conversion efficiency of the power conversion circuit.In addition, both the second end of the first common mode filtercapacitor and the second end of the second common mode filter capacitorare connected to the direct current side circuit, to provide a lowimpedance loop of a common mode current, thereby effectively reducing acommon mode leakage current from the power conversion circuit to anoutput port.

In a possible implementation, with reference to the second aspect, in afirst possible implementation, the low impedance circuit is a zeroimpedance circuit, or the low impedance circuit includes one resistor orat least two resistors in series.

In a possible implementation, with reference to the second aspect or thefirst possible implementation of the second aspect, in a second possibleimplementation, the filter circuit further includes a second powerinductor, the second power inductor includes a fifth winding and a sixthwinding, the second end of the third winding is connected to a first endof the fifth winding, a second end of the fourth winding is connected toa first end of the sixth winding, and a second end of the fifth windingand a second end of the sixth winding are connected to the alternatingcurrent side circuit.

In a possible implementation, with reference to the second aspect or thefirst or the second possible implementation of the second aspect, in athird possible implementation, the direct current side circuit includesa positive busbar, a busbar capacitor, and a negative busbar, and twoends of the busbar capacitor are respectively connected to the positivebusbar and the negative busbar.

In a possible implementation, with reference to the third possibleimplementation of the second aspect, in a fourth possibleimplementation, both the second end of the first common mode filtercapacitor and the second end of the second common mode filter capacitorare separately connected to the positive busbar.

In a possible implementation, with reference to the third possibleimplementation of the second aspect, in a fifth possible implementation,both the second end of the first common mode filter capacitor and thesecond end of the second common mode filter capacitor are separatelyconnected to the negative busbar.

In a possible implementation, with reference to the third possibleimplementation of the second aspect, in a sixth possible implementation,the second end of the first common mode filter capacitor is connected tothe positive busbar, and the second end of the second common mode filtercapacitor is connected to the negative busbar.

In a possible implementation, with reference to the third possibleimplementation of the second aspect, in a seventh possibleimplementation, the busbar capacitor includes a positive busbarcapacitor and a negative busbar capacitor, a first end of the positivebusbar capacitor is connected to the positive busbar, a second end ofthe positive busbar capacitor is connected to a first end of thenegative busbar capacitor, and a second end of the negative busbarcapacitor is connected to the negative busbar; and the second end of thefirst common mode filter capacitor and the second end of the secondcommon mode filter capacitor are connected to a middle point between thepositive busbar capacitor and the negative busbar capacitor.

In a possible implementation, with reference to any one of the secondaspect or the first to the seventh possible implementations of thesecond aspect, in an eighth possible implementation, the switchingnetwork includes a first converter bridge arm and a second converterbridge arm, the first converter bridge arm includes a first switchingdevice and a second switching device, and the second converter bridgearm includes a third switching device and a fourth switching device;

the control circuit controls the first switching device and the secondswitching device of the first converter bridge arm through a first sinemodulated wave and a first carrier;

the control circuit controls the third switching device and the fourthswitching device of the second converter bridge arm through a secondsine modulated wave and a second carrier; and

bipolar modulation is performed on the first carrier and the secondcarrier in a first preset angle range, unipolar modulation is performedin a second preset angle range, the first preset angle range is setbased on a direct current bias point of the first sine modulated wave orthe second sine modulated wave, the second preset angle range is anangle other than the first preset angle range in a sine wave period ofthe first sine modulated wave or the second sine modulated wave, and aswitching frequency of the bipolar modulation is higher than a switchingfrequency of the unipolar modulation.

In the eighth possible implementation, a direct current bias means thatan alternating current has a direct current component. The sinemodulated wave may have one direct current bias point. If a directcurrent bias value is 0, the direct current bias point is a zerocrossing point, or if a direct current bias value is not 0, a value ofthe direct current bias point is the direct current bias value. It canbe understood from the eighth possible implementation that a unipolarand bipolar hybrid modulation scheme is used, to further improveconversion efficiency of the power conversion circuit and further reducea leakage current.

In a possible implementation, with reference to the eighth possibleimplementation of the second aspect, in a ninth possible implementation,

the first preset angle range includes (−α, β), a value of −α and a valueof β are adjusted based on state information, and the state informationincludes a voltage of the positive busbar, a voltage of the negativebusbar, and a voltage of the alternating current side circuit.

According to a third aspect, a power conversion circuit is provided, andmay include:

a switching network, a control circuit, a filter circuit, a directcurrent side circuit, and an alternating current side circuit, where

the switching network is connected to the direct current side circuit,the switching network is connected to the control circuit, the switchingnetwork is connected to the filter circuit, and the filter circuit isconnected to the alternating current side circuit;

the control circuit is configured to control the switching network;

the filter circuit includes a third power inductor, a fourth powerinductor, a common mode choke, a first differential mode filtercapacitor, a first common mode filter capacitor, and a second commonmode filter capacitor;

the common mode choke includes a third winding and a fourth winding;

a first end of the third power inductor and a first end of the fourthpower inductor are separately connected to the switching network, asecond end of the third power inductor is connected to a first end ofthe third winding, and a second end of the fourth power inductor isconnected to a first end of the fourth winding;

two ends of the first differential mode filter capacitor arerespectively connected to the second end of the third power inductor andthe second end of the fourth power inductor;

a first end of the first common mode filter capacitor is connected to asecond end of the third winding, and a second end of the first commonmode filter capacitor is connected to the direct current side circuit byusing a low impedance circuit; and

a first end of the second common mode filter capacitor is connected to asecond end of the fourth winding, and a second end of the second commonmode filter capacitor is connected to the direct current side circuit byusing a low impedance circuit.

The low impedance circuit in a case in which the second end of the firstcommon mode filter capacitor is connected to the direct current sidecircuit by using the low impedance circuit may be referred to as a firstlow impedance circuit. The low impedance circuit in a case in which thesecond end of the second common mode filter capacitor is connected tothe direct current side circuit by using the low impedance circuit maybe referred to as a second low impedance circuit. The first lowimpedance circuit and the second low impedance circuit may be a same lowimpedance circuit, or may be different low impedance circuits.

It can be understood from the third aspect that, before the firstdifferential mode filter capacitor is connected to the common mode chokein the filter circuit, a loss caused when a high-frequency currentcomponent flows into the common mode choke can be effectively avoided,thereby improving conversion efficiency of the power conversion circuit.In addition, both the second end of the first common mode filtercapacitor and the second end of the second common mode filter capacitorare connected to the direct current side circuit, to provide a lowimpedance loop of a common mode current, thereby effectively reducing acommon mode leakage current from the power conversion circuit to anoutput port.

A difference between the third aspect of this application and the firstaspect is that the third power inductor and the fourth power inductorare used to implement functions of the first winding and the secondwinding. Actually, the first winding and the second winding may also beunderstood as two power inductors, and the solutions provided in thefirst aspect and the third aspect are essentially the same.

Other possible implementations of the third aspect may be understoodwith reference to any possible implementation of the first aspect.

According to a fourth aspect, a hybrid modulation control method isprovided. The method is applied to a power conversion circuit, and thepower conversion circuit includes a switching network, a controlcircuit, a filter circuit, a direct current side circuit, and analternating current side circuit.

The switching network is connected to the direct current side circuit,the switching network is connected to the control circuit, the switchingnetwork is connected to the filter circuit, the filter circuit isconnected to the alternating current side circuit, the switching networkincludes a first converter bridge arm and a second converter bridge arm,the first converter bridge arm includes a first switching device and asecond switching device, and the second converter bridge arm includes athird switching device and a fourth switching device.

The method includes:

The control circuit controls the first switching device and the secondswitching device of the first converter bridge arm through a first sinemodulated wave and a first carrier.

The control circuit controls the third switching device and the fourthswitching device of the second converter bridge arm through a secondsine modulated wave and a second carrier.

Bipolar modulation is performed on the first carrier and the secondcarrier in a first preset angle range, unipolar modulation is performedin a second preset angle range, the first preset angle range is setbased on a direct current bias point of the first sine modulated wave orthe second sine modulated wave, the second preset angle range is anangle other than the first preset angle range in a sine wave period ofthe first sine modulated wave or the second sine modulated wave, and aswitching frequency of the bipolar modulation is higher than a switchingfrequency of the unipolar modulation.

It can be understood from the fourth aspect that a unipolar and bipolarhybrid modulation scheme is used, to improve conversion efficiency ofthe power conversion circuit and reduce a leakage current.

With reference to the fourth aspect, in a first possible implementation,the first preset angle range includes (−α, β), a value of −α and a valueof β are adjusted based on state information, and the state informationincludes a voltage of a positive busbar, a voltage of a negative busbar,and a voltage of the alternating current side circuit.

According to a fifth aspect, an inverter is provided, and includes thepower conversion circuit according to any one of the first aspect or thepossible implementations of the first aspect, any one of the secondaspect or the possible implementations of the second aspect, or any oneof the third aspect or the possible implementations of the third aspect.

According to a sixth aspect, a photovoltaic power generation system isprovided, and may include:

a photovoltaic panel, an inverter, and an alternating current network,where

the photovoltaic panel is connected to the inverter, and the inverter isconnected to the alternating current network;

the photovoltaic panel is configured to convert light energy into adirect current;

the inverter includes the power conversion circuit according to any oneof the first aspect or the possible implementations of the first aspect,any one of the second aspect or the possible implementations of thesecond aspect, or any one of the third aspect or the possibleimplementations of the third aspect, and is configured to convert thedirect current into an alternating current; and

the alternating current network is configured to transmit thealternating current.

According to a seventh aspect, a computer-readable storage medium. Thecomputer-readable storage medium stores instructions, and when theinstructions are run on a computer, the computer is enabled to performthe control method according to the fourth aspect. According to thesolutions provided in the embodiments, before the first differentialmode filter capacitor is connected to the common mode choke in thefilter circuit, the loss caused when the high-frequency currentcomponent flows into the common mode choke can be effectively avoided,thereby improving conversion efficiency of the power conversion circuit.In addition, both the second end of the first common mode filtercapacitor and the second end of the second common mode filter capacitorare connected to the direct current side circuit, to provide the lowimpedance loop of the common mode current, thereby effectively reducingthe common mode leakage current from the power conversion circuit to theoutput port.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a scenario to which an embodiment isapplied;

FIG. 2 is a schematic diagram of another scenario to which an embodimentis applied;

FIG. 3 is a schematic diagram of a power conversion circuit according toan embodiment;

FIG. 4 is a structural diagram of a power conversion circuit accordingto an embodiment;

FIG. 5 is a structural diagram of another power conversion circuitaccording to an embodiment;

FIG. 6 is a structural diagram of another power conversion circuitaccording to an embodiment;

FIG. 7 is a structural diagram of another power conversion circuitaccording to an embodiment;

FIG. 8 is a structural diagram of another power conversion circuitaccording to an embodiment;

FIG. 9 is a structural diagram of another power conversion circuitaccording to an embodiment;

FIG. 10 is a structural diagram of another power conversion circuitaccording to an embodiment;

FIG. 11 is a diagram of a waveform of a unipolar modulation-basedcircuit according to an embodiment;

FIG. 12 is a schematic diagram of a waveform of a hybrid modulationcontrol method according to an embodiment;

FIG. 13 is a diagram of a waveform of a hybrid modulation-based circuitaccording to an embodiment;

FIG. 14 is a diagram of a comparison between current conversionefficiency in different control manners according to an embodiment; and

FIG. 15 is a diagram of a waveform of an inverter voltage and a waveformof a current of a common mode choke according to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments provide a power conversion circuit, to improve conversionefficiency of an inverter, and effectively reduce a common mode leakagecurrent. The embodiments of this application further provide acorresponding inverter and a corresponding hybrid modulation controlmethod.

To make a person of ordinary skill in the art understand solutions inthe embodiments better, the following describes the solutions in theembodiments with reference to accompanying drawings. The describedembodiments are merely some, rather than all, of the embodiments.

In the embodiments and accompanying drawings, the terms such as “first”and “second” are intended to distinguish between similar objects but donot necessarily indicate a specific order or sequence. It should beunderstood that the data termed in such a way may be changed in propercircumstances, so that the embodiments described herein can beimplemented in other orders than the order illustrated or describedherein. In addition, the terms “include” and “contain” and any othervariants mean to cover the non-exclusive inclusion, for example, aprocess, method, system, product, or device that includes a list ofsteps or units is not necessarily limited to those steps or units, butmay include other steps or units not expressly listed or inherent tosuch a process, method, system, product, or device.

The embodiments provide a power conversion circuit, an inverter, and ahybrid modulation control method. The power conversion circuit, theinverter, and the hybrid modulation control method may be applied to ascenario architecture shown in FIG. 1.

FIG. 1 is a schematic diagram of a scenario to which an embodiment isapplied.

FIG. 1 shows an architecture of an uninterruptible power system (UPS).In a normal case, mains electricity is used to directly supply power toa load, and power-frequency alternating current mains electricity maypass through a direct current (DC) inverter/an alternating current (AC)inverter to be converted from an alternating current to a directcurrent, and then pass through a DC/DC converter to charge a battery.When the mains electricity is faulty, the battery supplies power to theload after the DC/DC inverter and the DC/AC inverter convert the directcurrent into the alternating current. The power conversion circuit andthe hybrid modulation control method provided in this application may beapplied to a running process of an AC/DC rectifier or a DC/AC inverter.

In another possible scenario, the power conversion circuit, theinverter, and the control method provided in the embodiments may beapplied to a grid-connected photovoltaic power generation system.

FIG. 2 is a schematic diagram of another scenario to which an embodimentis applied.

As shown in FIG. 2, a direct current output by a photovoltaic panelpasses through a DC/AC inverter, and the direct current is convertedinto an alternating current, to implement grid-connected powergeneration of the photovoltaic panel. The power conversion circuit andthe hybrid modulation control method provided in this application may beapplied to a running process of the DC/AC inverter.

Based on the foregoing scenarios, the following further describes thesolutions by using embodiments.

FIG. 3 is a schematic diagram of a power conversion circuit according toan embodiment.

As shown in FIG. 3, the power conversion circuit may include a directcurrent side circuit 701, a switching network 702, a control circuit703, a filter circuit 704, and an alternating current side circuit 705.

The switching network 702 is connected to the direct current sidecircuit 701, the switching network 702 is connected to the controlcircuit 703, the switching network 702 is connected to the filtercircuit 704, and the filter circuit 704 is connected to the alternatingcurrent side circuit 705.

The control circuit 703 is configured to control the switching network702 to convert, into an alternating current according to a presetmodulation scheme, a direct current input by the direct current sidecircuit 701. The filter circuit 704 is configured to: filter out ahigh-frequency ripple generated by the switching network 702 in amodulation process, and transmit a processed alternating current to thealternating current side circuit 705.

In a possible circuit structure, a structural diagram shown in FIG. 4may be used for the power conversion circuit shown in FIG. 3.

FIG. 4 is a structural diagram of a power conversion circuit accordingto an embodiment.

The direct current side circuit shown in FIG. 4 includes a positivebusbar, a negative busbar, and a busbar capacitor.

The switching network may include a first converter bridge arm and asecond converter bridge arm, the first converter bridge arm includes afirst switching device T1 and a second switching device T2, and thesecond converter bridge arm includes a third switching device T3 and afourth switching device T4.

Optionally, an insulated gate bipolar transistor (IGBT), gallium nitride(GaN), a metal-oxide semiconductor field-effect transistor (, MOSFET),or another power semiconductor device may be selected as the firstswitching device T1, the second switching device T2, the third switchingdevice T3, and the fourth switching device T4.

The filter circuit may include a first power inductor Lal, a common modechoke Lcm, a first differential mode filter capacitor Cdm, and a firstcommon mode filter capacitor Ccm1. In a possible implementation, thefilter circuit may further include a second common mode filter capacitorCcm2.

The first power inductor Lal includes a first winding Lall and a secondwinding La12. It may be understood that a coil quantity ratio of thefirst winding Lall to the second winding La12 may be adjusted based on arequirement. In a possible scenario, a same coil quantity may be used.Certainly, a specific coil quantity is determined based on an actualscenario. This is not limited in this embodiment of this application.The common mode choke Lcm includes a third winding Lcmll and a fourthwinding Lcm12.

A first end of the first winding Lall and a first end of the secondwinding La12 are separately connected to a point A of the switchingnetwork, a second end of the first winding La11 is connected to a firstend of the third winding Lcmll, and a second end of the second windingLa12 is connected to a first end of the fourth winding Lcm12.

A first end of the first differential mode filter capacitor Cdm isconnected to the second end of the first winding Lall, and a second endof the first differential mode filter capacitor Cdm is connected to thesecond end of the second winding La12.

A first end of the first common mode filter capacitor Ccm1 is connectedto a second end of the third winding Lcmll, and a second end of thefirst common mode filter capacitor Ccm1 is connected to the directcurrent side circuit by using a low impedance circuit. In thisembodiment, the low impedance circuit may be understood as a conductingwire that is used to directly connect the second end of the first commonmode filter capacitor Ccm1 and the direct current side circuit. In thisscenario, the low impedance circuit is a zero impedance circuit.

A first end of the second common mode filter capacitor Ccm2 is connectedto a second end of the fourth winding Lcm12, and a second end of thesecond common mode filter capacitor Ccm2 is connected to the directcurrent side circuit by using a low impedance circuit. In thisembodiment, the low impedance circuit may be understood as a conductingwire that is used to directly connect the second end of the secondcommon mode filter capacitor Ccm2 and the direct current side circuit.In this scenario, the low impedance circuit is a zero impedance circuit.

A common mode leakage current is drawn back to the direct current sidecircuit by using the first common mode filter capacitor Ccm1 and thesecond common mode filter capacitor Ccm2, to provide a low impedanceloop of the common mode leakage current, so that a ground leakagecurrent of an inverter can be significantly reduced. In addition, in acircuit structure shown in FIG. 4, the first power inductor Lal includesthe two windings La11 and La12, to provide common mode impedance, reduceimpact of a common mode voltage of the inverter, and reduce a requiredcommon mode choke.

It should be noted that, in the foregoing solution, the first powerinductor Lal may alternatively be only one winding, and does not need tobe divided into the first winding and the second winding. In this case,a first end of the first power inductor is connected to the switchingnetwork, a second end of the first power inductor is connected to thefirst end of the third winding, and the first end of the fourth windingis connected to the switching network; and the first end of the firstdifferential mode filter capacitor is connected to the second end of thefirst power inductor, and the second end of the first differential modefilter capacitor is connected to the first end of the third winding.

Optionally, the first power inductor Lal may alternatively include twoindependent power inductors, and this may be understood with referenceto FIG. 5.

FIG. 5 is a structural diagram of another power conversion circuitaccording to an embodiment.

In comparison with FIG. 4, in the embodiment shown in FIG. 5, the firstpower inductor Lal in FIG. 4 is replaced with a third power inductor La3and a fourth power inductor La4. It may be understood that a coilquantity of the third power inductor La3 and a coil quantity of thefourth power inductor La4 may be selected with reference to the firstwinding La11 and the second winding La12, or may be adjusted based on arequirement. A specific coil quantity is determined based on an actualscenario.

Optionally, to further reduce impact of a high-frequency ripple from theinverter on an alternating current port, a second power inductor La2 maybe further added between the filter circuit and the alternating currentside circuit, to provide impedance and reduce impact of the powerconversion circuit on the alternating current port. This may beunderstood with reference to FIG. 6.

FIG. 6 is a structural diagram of another power conversion circuitaccording to an embodiment.

As shown in FIG. 6, the second power inductor La2 includes a fifthwinding La21 and a sixth winding La22, the second end of the thirdwinding Lcml1 is connected to a first end of the fifth winding La21, asecond end of the fourth winding Lcm12 is connected to a first end ofthe sixth winding La22, and a second end of the fifth winding La21 and asecond end of the sixth winding La21 are connected to the alternatingcurrent side circuit. It may be understood that a coil quantity ratio ofthe fifth winding La21 to the sixth winding La22 may be adjusted basedon a requirement. In a possible scenario, a same coil quantity may beused.

Based on the foregoing circuit, it may be found that, in thisembodiment, the common mode leakage current is drawn back to the directcurrent side circuit by using the first common mode filter capacitorCcm1 and the second common mode filter capacitor Ccm2, to provide thelow impedance loop of the common mode leakage current, so that theground leakage current of the inverter can be significantly reduced.However, the direct current side circuit that serves as a target towhich the common mode leakage current is drawn back includes thepositive busbar and the negative busbar. It may be understood that thetarget to which the common mode leakage current is drawn back mayinclude the following plurality of cases. The cases are described belowwith reference to accompanying drawings.

It should be noted that changing a case in which the common mode leakagecurrent is drawn back to the direct current side circuit does not affecta solution in which a separate inductor is used as the first powerinductor La1 or a solution in which the second power inductor La2 isadded between the filter circuit and the alternating current sidecircuit. In other words, according to the following solutions, thesolution in which a separate inductor is used as the first powerinductor Lal or the solution in which the second power inductor La2 isadded between the filter circuit and the alternating current sidecircuit may be combined or removed. The following provides descriptionsby using the circuit shown in FIG. 6 as an example. A specific circuitchange is determined based on an actual scenario, and is not limitedherein.

1. The common mode leakage current is drawn back to the negative busbar.

In this embodiment, refer to structural diagrams in FIG. 4 to FIG. 10.Details are not described herein again.

2. The common mode leakage current is drawn back to the positive busbar.

In this embodiment, refer to a structural diagram of another powerconversion circuit shown in FIG. 7. As shown in FIG. 7, the second endof the first common mode filter capacitor Ccm1 is connected to thepositive busbar, and the second end of the second common mode filtercapacitor Ccm2 is connected to the positive busbar. Descriptions ofother parts may be understood with reference to related content in FIG.4 to FIG. 10. Details are not described herein again.

3. The common mode leakage current is drawn back to the positive busbarand the negative busbar.

In this embodiment, refer to a structural diagram of another powerconversion circuit shown in FIG. 8. As shown in FIG. 8, the second endof the first common mode filter capacitor Ccm1 is connected to thepositive busbar, and the second end of the second common mode filtercapacitor Ccm2 is connected to the negative busbar, so that completelyequivalent impedance circuits are provided for the two windings of thecommon mode choke Lcm, and a one-sided saturation phenomenon is notprone to occur in the common mode choke. Descriptions of other parts maybe understood with reference to related content in FIG. 4 to FIG. 10.Details are not described herein again.

4. The common mode leakage current is drawn back to a middle pointbetween the positive busbar and the negative busbar.

In this embodiment, refer to a structural diagram of another powerconversion circuit shown in FIG. 9. As shown in FIG. 9, the busbarcapacitor Cdc in this embodiment includes a positive busbar capacitorCdcp and a negative busbar capacitor Cdcn. A first end of the positivebusbar capacitor Cdcp is connected to the positive busbar, a second endof the positive busbar capacitor Cdcp is connected to a first end of thenegative busbar capacitor Cdcn, a second end of the negative busbarcapacitor Cdcn is connected to the negative busbar, and there is abusbar middle point M between the positive busbar capacitor Cdcp and thenegative busbar capacitor Cdcn. The second end of the first common modefilter capacitor Ccm1 and the second end of the second common modefilter capacitor Ccm2 are connected to the busbar middle point M, sothat a middle point between output common mode capacitors Lcm can beconnected to the busbar middle point M. In this filter manner, a lowimpedance circuit of the common mode leakage current may also beprovided, and an output common mode leakage current of the inverter issignificantly reduced.

In addition, in this filter manner, completely equivalent impedancecircuits are also provided for the two windings of the common mode chokeLcm, and a one-sided saturation phenomenon is not prone to occur in thecommon mode choke Lcm. Descriptions of other parts may be understoodwith reference to related content in FIG. 4 to FIG. 10. Details are notdescribed herein again.

Optionally, a low impedance resistor R1 may be connected to a conductingwire for drawing the common mode leakage current back to the directcurrent side circuit, to serve as a low impedance circuit. As shown in astructural diagram of another power conversion circuit in FIG. 10, a lowimpedance resistor R1 is connected between the busbar middle point M andthe filter circuit. In other words, there is a resistor R1 in the lowimpedance circuit. The low impedance resistor R1 may provide damping ina common mode loop, to effectively suppress oscillation in the commonmode loop. In addition, a parameter of the low impedance resistance R1may be adjusted based on an actual measurement case, to meet differentcircuit conditions. Further, in this embodiment, descriptions areprovided by using an example in which only one low impedance resistor R1is added. However, R1 may alternatively be replaced with two or moreresistors in series.

It may be understood that a connection of the low impedance resistor R1may also be applied to the foregoing solutions 1 to 4 of a bus barconnection point. For a specific connection manner, refer to relatedcontent in FIG. 10. Details are not described herein again.

In this embodiment, the control circuit needs to control a switchingfrequency of the switching network according to a control policy, todraw the common mode leakage current back to the direct current sidecircuit, thereby providing the low impedance circuit of the common modeleakage current, and significantly reducing the ground leakage currentof the inverter. The control policy may be based on unipolar modulation,or may be based on unipolar and bipolar hybrid modulation.

In a possible scenario based on the unipolar modulation, a waveformshown in FIG. 11 may be obtained through measurement based on thecircuit shown in FIG. 6. FIG. 11 is a diagram of a waveform of aunipolar modulation-based circuit according to an embodiment. It can beunderstood from FIG. 11 that in the foregoing circuit, although theunipolar modulation-based control policy can be used to reduce outputpower inductance, the unipolar modulation leads to an extremely severecommon mode voltage at a zero crossing point of a voltage. The commonmode voltage leads to a high voltage second of the common mode choke, alarge size of the required common mode choke, and high costs. Inaddition, the common mode voltage leads to an extremely serious highfrequency loss on the common mode choke, and a significant reduction inan advantage of the unipolar modulation.

To use the advantage of the unipolar modulation, and reduce a size ofthe common mode choke in the provided circuit, an embodiment provides ahybrid modulation control method. The control method is implemented byusing the control circuit 703. For example, the control circuit 703controls the first switching device T1 and the second switching deviceT2 of the first converter bridge arm through a first sine modulated waveand a first carrier. The control circuit controls the third switchingdevice T3 and the fourth switching device T4 of the second converterbridge arm through a second sine modulated wave and a second carrier.

In this embodiment, the switching frequency of the switching network maybe controlled by using a unipolar and bipolar hybrid modulation schemeshown in FIG. 12. FIG. 12 is a schematic diagram of a waveform of ahybrid modulation control method according to an embodiment.

As shown in FIG. 12, bipolar modulation is performed on the firstcarrier and the second carrier in a first preset angle range (−α, β),and unipolar modulation is performed in a second preset angle range,namely, an angle range obtained after (−α, β) is subtracted from 360degrees. A period of 360 degrees is used as an example. It may also beunderstood that the bipolar modulation is used as a modulation scheme inangle ranges (0−β), ((π−α)−π). (π−(π+β)), and (2π−α)−2π), and theunipolar modulation is used in angles (β−(π−α)) and ((π+β)−(2π−α)). Thefirst preset angle range is set based on a direct current bias point ofthe first sine modulated wave or the second sine modulated wave, thesecond preset angle range is an angle other than the first preset anglerange in a sine wave period of the first sine modulated wave or thesecond sine modulated wave, a switching frequency of the bipolarmodulation is higher than a switching frequency of the unipolarmodulation, and the switching frequency of the bipolar modulation is twotimes of the switching frequency of the unipolar modulation. In somepossible scenarios, the frequency of the bipolar modulation may be setto several times of the switching frequency of the unipolar modulationbased on a current ripple condition of the first power inductor La1, anda possible range is from three times to 15 times. In the modulationscheme, a smooth transition between the unipolar modulation and thebipolar modulation can be implemented, and a current of a filterinductor is not distorted, so that circuit oscillation andelectromagnetic compatibility (EMC) problems caused by a currentdistortion can be suppressed.

It should be noted that, the direct current bias point may be any value.Descriptions are provided with reference to an experimental measurementscenario by using a zero point as the direct current bias point. Inother words, different direct current biases may be obtained based ondifferent experimental scenarios. This is not limited herein. Inaddition, the first sine modulated wave or the second sine modulatedwave that is set at the direct current bias point may be symmetrical.There may be one or more first sine modulated waves or second sinemodulated waves, and a specific quantity is determined based on anactual scenario, and is not limited herein.

Optionally, after α and β at a current moment are set and controlswitching in one period is completed, switching between the unipolarmodulation and the bipolar modulation may be implemented by adjusting acarrier count frequency and a count change of a counter.

It may be understood that a value of α and a value of β are adjustedbased on state information. To be specific, α and β may be the same ormay be different. The state information includes a voltage of thepositive busbar, a voltage of the negative busbar, and a voltage of thealternating current side circuit.

In a possible scenario, a may be set to 30°, and β may be set to 30°. Inthis case, a waveform shown in FIG. 13 may be obtained throughmeasurement. FIG. 13 is a diagram of a waveform of a hybridmodulation-based circuit according to an embodiment of this application.In comparison with a result shown in FIG. 11, it can be found that anamplitude value of a current drawn back to a busbar obviously decreasesat a zero crossing point of an alternating current voltage, and there isno saturation phenomenon. In other words, a same common mode chokecannot meet a normal working requirement of the inverter in a case ofthe unipolar modulation, but a normal working of the inverter can beensured in the control method provided in this application. The controlmethod is combined with the circuit structures provided in thisapplication, so that efficiency of the inverter can be significantlyimproved, a size of a passive device can be reduced, and the common modeleakage current can be effectively controlled.

According to the foregoing control method, FIG. 14 may be obtained. FIG.14 is a diagram of a comparison between current conversion efficiency indifferent control manners according to an embodiment. Compared with thebipolar modulation, the unipolar modulation leads to a severe commonmode voltage at a zero crossing point of a power-frequency voltage. Witha low load, there is an extremely severe high frequency loss on thecommon mode choke, and relatively low efficiency. With an increase inoutput power, an advantage in halving a semiconductor switching loss inthe unipolar modulation is reflected, and efficiency of the inverter isincreased. Curves in FIG. 14 show that in comparison with the unipolarmodulation and the bipolar modulation, advantages of the unipolarmodulation and the bipolar modulation can be fully used according to ahybrid modulation control policy provided in this embodiment of thisapplication, to not only reduce a semiconductor loss, but also reduce arequired common mode choke.

In a possible scenario, a data diagram shown in FIG. 15 may be obtainedbased on the circuit in FIG. 6 by using the control method. FIG. 15 is adiagram of a waveform of an inverter voltage and a waveform of a currentof a common mode choke according to an embodiment. It can be obviouslyseen that there is no current with a high-frequency ripple on a windingof the common mode choke of the circuit provided in this application, sothat a high-frequency loss on the common mode choke can be reduced, andperformance of the inverter is further improved. In addition, the firstpower inductor Lal and the second power inductor La2 in the circuitstructure provided in this application may include two windings, toprovide common mode impedance, reduce impact of a common mode voltage ofthe inverter, and reduce a requirement for the common mode choke.

The inverter provided in this embodiment is a device that includes theforegoing described power conversion circuit. This may be understoodwith reference to the foregoing descriptions of the power conversioncircuit.

A person of ordinary skill in the art may understand that some or allsteps of various circuit operations in the foregoing embodiments may beimplemented by a program instructing related hardware. The program maybe stored in a computer-readable storage medium. The storage medium mayinclude any medium that can store program code, for example, a USB flashdrive, a removable hard disk, a read-only memory (ROM), a random accessmemory (RAM), a magnetic disk, or a compact disc.

The power conversion circuit, the inverter, and the hybrid modulationcontrol method provided in the embodiments are described in detailabove. The principle and implementation of the embodiments are describedherein through specific examples. The description about the embodimentsis merely provided to help understand the method and core ideas of theembodiments. In addition, the person of ordinary skill in the art canmake modifications to the specific implementations and applicationscopes according to the ideas of the embodiments. Therefore the contentof the embodiments shall not be construed as limiting.

What is claimed is:
 1. A power conversion circuit, comprising: a switching network, a control circuit, a filter circuit, a direct current side circuit, and an alternating current side circuit, wherein the switching network is connected to the direct current side circuit, the switching network is connected to the control circuit, the switching network is connected to the filter circuit, and the filter circuit is connected to the alternating current side circuit; the control circuit is configured to control the switching network; the filter circuit comprises a first power inductor, a common mode choke, a first differential mode filter capacitor, a first common mode filter capacitor, and a second common mode filter capacitor; the first power inductor comprises a first winding and a second winding, and the common mode choke comprises a third winding and a fourth winding; both a first end of the first winding and a first end of the second winding are separately connected to the switching network, a second end of the first winding is connected to a first end of the third winding, and a second end of the second winding is connected to a first end of the fourth winding; two ends of the first differential mode filter capacitor are respectively connected to the second end of the first winding and the second end of the second winding; a first end of the first common mode filter capacitor is connected to a second end of the third winding, and a second end of the first common mode filter capacitor is connected to the direct current side circuit by using a low impedance circuit; and a first end of the second common mode filter capacitor is connected to a second end of the fourth winding, and a second end of the second common mode filter capacitor Ccm2 is connected to the direct current side circuit by using a low impedance circuit.
 2. The power conversion circuit according to claim 1, wherein the low impedance circuit is a zero impedance circuit, or the low impedance circuit comprises one resistor or at least two resistors in series.
 3. The power conversion circuit according to claim 1, wherein the filter circuit further comprises a second power inductor, the second power inductor comprises a fifth winding and a sixth winding, the second end of the third winding is connected to a first end of the fifth winding, the second end of the fourth winding is connected to a first end of the sixth winding, and a second end of the fifth winding and a second end of the sixth winding are connected to the alternating current side circuit.
 4. The power conversion circuit according to claim 1, wherein the direct current side circuit comprises a positive busbar, a busbar capacitor, and a negative busbar, and two ends of the busbar capacitor are respectively connected to the positive busbar and the negative busbar.
 5. The power conversion circuit according to claim 4, wherein both the second end of the first common mode filter capacitor and the second end of the second common mode filter capacitor are separately connected to the positive busbar.
 6. The power conversion circuit according to claim 4, wherein both the second end of the first common mode filter capacitor and the second end of the second common mode filter capacitor are separately connected to the negative busbar.
 7. The power conversion circuit according to claim 4, wherein the second end of the first common mode filter capacitor is connected to the positive busbar, and the second end of the second common mode filter capacitor is connected to the negative busbar.
 8. The power conversion circuit according to claim 4, wherein the busbar capacitor comprises a positive busbar capacitor and a negative busbar capacitor, a first end of the positive busbar capacitor is connected to the positive busbar, a second end of the positive busbar capacitor is connected to a first end of the negative busbar capacitor, and a second end of the negative busbar capacitor is connected to the negative busbar; and the second end of the first common mode filter capacitor and the second end of the second common mode filter capacitor are connected to a middle point between the positive busbar capacitor and the negative busbar capacitor.
 9. The power conversion circuit according to claim 1, wherein the switching network comprises a first converter bridge arm and a second converter bridge arm, the first converter bridge arm comprises a first switching device and a second switching device, and the second converter bridge arm comprises a third switching device and a fourth switching device; the control circuit controls the first switching device and the second switching device of the first converter bridge arm through a first sine modulated wave and a first carrier; the control circuit controls the third switching device and the fourth switching device of the second converter bridge arm through a second sine modulated wave and a second carrier; and bipolar modulation is performed on the first carrier and the second carrier in a first preset angle range, unipolar modulation is performed in a second preset angle range, the first preset angle range is set based on a direct current bias point of the first sine modulated wave or the second sine modulated wave, the second preset angle range is an angle other than the first preset angle range in a sine wave period of the first sine modulated wave or the second sine modulated wave, and a switching frequency of the bipolar modulation is higher than a switching frequency of the unipolar modulation.
 10. The power conversion circuit according to claim 9, wherein the first preset angle range comprises (−α, β), a value of −α and a value of β are adjusted based on state information, and the state information comprises a voltage of the positive busbar, a voltage of the negative busbar, and a voltage of the alternating current side circuit.
 11. A hybrid modulation control method, the method applied to a power conversion circuit, and the power conversion circuit comprises a switching network, a control circuit, a filter circuit, a direct current side circuit, and an alternating current side circuit; the switching network is connected to the direct current side circuit, the switching network is connected to the control circuit, the switching network is connected to the filter circuit, the filter circuit is connected to the alternating current side circuit, the switching network comprises a first converter bridge arm and a second converter bridge arm, the first converter bridge arm comprises a first switching device and a second switching device, and the second converter bridge arm comprises a third switching device and a fourth switching device; and the method comprises: controlling, by the control circuit, the first switching device and the second switching device of the first converter bridge arm through a first sine modulated wave and a first carrier; and controlling, by the control circuit, the third switching device and the fourth switching device of the second converter bridge arm through a second sine modulated wave and a second carrier, wherein bipolar modulation is performed on the first carrier and the second carrier in a first preset angle range, unipolar modulation is performed in a second preset angle range, the first preset angle range is set based on a direct current bias point of the first sine modulated wave or the second sine modulated wave, the second preset angle range is an angle other than the first preset angle range in a sine wave period of the first sine modulated wave or the second sine modulated wave, and a switching frequency of the bipolar modulation is higher than a switching frequency of the unipolar modulation.
 12. The control method according to claim 11, wherein the first preset angle range comprises (−α, β), a value of −α and a value of β are adjusted based on state information, and the state information comprises a voltage of the positive busbar, a voltage of a negative busbar, and a voltage of the alternating current side circuit.
 13. A photovoltaic power generation system, comprising: a photovoltaic panel, an inverter, and an alternating current network, wherein the photovoltaic panel is connected to the inverter, and the inverter is connected to the alternating current network; the photovoltaic panel is configured to convert light energy into a direct current; the inverter comprises the power conversion circuit, and is configured to convert the direct current into an alternating current; and the alternating current network is configured to transmit the alternating current; wherein the power conversion circuit comprises: a switching network, a control circuit, a filter circuit, a direct current side circuit, and an alternating current side circuit, wherein the switching network is connected to the direct current side circuit, the switching network is connected to the control circuit, the switching network is connected to the filter circuit, and the filter circuit is connected to the alternating current side circuit; the control circuit is configured to control the switching network; the filter circuit comprises a first power inductor, a common mode choke, a first differential mode filter capacitor, a first common mode filter capacitor, and a second common mode filter capacitor; the first power inductor comprises a first winding and a second winding, and the common mode choke comprises a third winding and a fourth winding; both a first end of the first winding and a first end of the second winding are separately connected to the switching network, a second end of the first winding is connected to a first end of the third winding, and a second end of the second winding is connected to a first end of the fourth winding; and two ends of the first differential mode filter capacitor are respectively connected to the second end of the first winding and the second end of the second winding.
 14. The system according to claim 13, wherein a first end of the first common mode filter capacitor is connected to a second end of the third winding, and a second end of the first common mode filter capacitor is connected to the direct current side circuit by using a low impedance circuit; and a first end of the second common mode filter capacitor is connected to a second end of the fourth winding, and a second end of the second common mode filter capacitor is connected to the direct current side circuit by using a low impedance circuit the low impedance circuit is a zero impedance circuit, or the low impedance circuit comprises one resistor or at least two resistors in series.
 15. The system according to claim 14, wherein the filter circuit further comprises a second power inductor, the second power inductor comprises a fifth winding and a sixth winding, the second end of the third winding is connected to a first end of the fifth winding, the second end of the fourth winding is connected to a first end of the sixth winding, and a second end of the fifth winding and a second end of the sixth winding are connected to the alternating current side circuit.
 16. The system according to claim 14, wherein the direct current side circuit comprises a positive busbar, a busbar capacitor, and a negative busbar, and two ends of the busbar capacitor are respectively connected to the positive busbar and the negative busbar.
 17. The system according to claim 16, wherein both the second end of the first common mode filter capacitor and the second end of the second common mode filter capacitor are separately connected to the positive busbar.
 18. The system according to claim 16, wherein both the second end of the first common mode filter capacitor and the second end of the second common mode filter capacitor are separately connected to the negative busbar. 